The planetary gear train of the internal engagement type is known in the art. For example, the planetary gear train of the internal engagement type is used in a speed decreasing device, a speed increasing device and so on. The planetary gear train of the internal engagement type has an internal gear and an external gear, which is internally engaged with the internal gear.
In each of the internal gear and the external gear for the planetary gear train of the internal engagement type, which is disclosed in Japanese Patent Publication No. 2016-65579 (corresponding to U.S. Pat. No. 9,856,945 B2), a run-out portion is formed at a tooth-front portion of a gear tooth. A possible contact between the tooth-front portion of the internal gear and the tooth-front portion of the external gear is avoided by the above run-out portion in a teeth non-engagement area, in which the gear tooth of the internal gear and the gear tooth of the external gear are out of engagement between them.
The above prior art, however, does not disclose a definite design standard for a size of the run-out portion of the external gear or the internal gear. A contact surface length of the gear tooth between the external gear and the internal gear may be decreased depending on an amount of the run-out portion. Durability of the planetary gear train is thereby decreased.